Breast cancer is a leading cause of cancer-related deaths in American women. Despite advances in early detection and treatment, mortality for those 20% of patients with recurrences and/or metastases is nearly 100%. Distinct sets of genes and proteins dictate progression from precursor lesions, to invasive carcinoma and finally to metastatic disease. Characterizing genes that regulate the growth and metastatic ability of breast cancer may identify novel biomarkers to help clinicians guide current treatments, and may offer novel targets for therapeutic intervention. We recently identified EZH2 (enhancer of Zeste homolog 2), a polycomb group protein (PcG), as being up-regulated in invasive and metastatic breast cancer. In Drosophila, PcG proteins have been shown to maintain gene expression programs during development by negatively regulating target loci including homeotic genes. The role of EZH2 in breast cancer has not been investigated. Our central hypothesis is that dysregulated expression of the PcG protein EZH2 promotes the growth and invasion of breast cancer by transcriptional silencing of specific target genes. Our long-term goal is to functionally characterize EZH2 and to elucidate its role as a novel biomarker of metastatic potential in breast cancer. The objective of this application is to begin to study the role of EZH2 in the development of breast cancer and delineate its clinical application. The specific aims of this project are as follows: 1. To determine the clinical utility of EZH2 as a novel biomarker of aggressive breast cancer in a large group of invasive carcinomas of the breast with clinical and treatment information, 2. To determine the role of EZH2 in regulating neoplastic characteristics in vitro and in vivo by overexpressing or inhibiting EZH2 expression in immortalized human mammary epithelial cells and in breast cancer cells, and by developing transgenic mice overexpressing EZH2 in the mammary epithelium, and 3. To identify and characterize transcriptional target genes of EZH2 that play a role in breast cancer progression by performing transcriptome analysis on immortalized mammary epithelial cells overexpressing EZH2 and in tissues of invasive carcinomas with and without EZH2 overexpression. In summary, this proposal addresses the clinical utility and functional role of EZH2 in the regulation of breast cancer progression.